Autocatalytic or electroless gold plating baths are widely employed for the development of gold deposits on both conductive and non-conductive substrates, particularly for electronics applications where optimum electrical properties in the deposit are desirable. To achieve the optimum electrical properties, it is desirable that the gold be of high purity, i.e., 99.9% or better, and that the deposit be substantially uniform over the surface of the workpiece.
Generally such electroless gold plating solutions have utilized alkali metal gold cyanide and free cyanide, and a water soluble borohydrate or an amine borane as the reducing agent. As the bath is replenished, the cyanide concentration increases, and this has an unfavorable effect upon the deposition rate and the stability of the composition. In an article by Martin Ulrich Kittel and Christoph Julius Raub entitled "Elektrochemische Stabilitaetsbestimmung Reduktiv Arbeitender Goldelektrolyte" published in Metalloberflaeche, Volume 41 (1987) at pages 309-313, there is discussed the effect of various compounds as stabilizers in gold plating compositions. None of the compounds reported by the authors serves effectively to provide a stable electroless gold plating bath which could be replenished a number of times without adverse effect upon its performance.
Accordingly, it is an object of the present invention to provide a novel and highly effective electroless gold plating composition which provides a useful rate of autocatalytic deposition of the gold upon the substrate, and which can be replenished a number of times without significantly adverse effect upon the plating rate or properties.
It is also an object to provide such an electroless gold plating composition which may be formulated readily and which is relatively stable in an industrial plating environment.
Another object is to provide a method for autocatalytic deposition of substantially pure gold upon a workpiece utilizing a relatively stable composition which can be replenished easily a number of times without significantly adverse effect upon the plating rate.